This invention relates to ionic compounds and methods for their preparation In particular the invention relates to ionic compounds which are liquid at relatively low temperatures, i.e. generally below about 100xc2x0 C., and preferably below about 60xc2x0 C. and more preferably which are liquid at or near to ambient temperature.
There is much current interest in the field of ionic liquids. Such systems, which are examples of molten salts, have a number of interesting and useful chemical properties, and have utility, for example, as highly polar solvents for use in preparative chemistry, and as catalysts. They also have particular application in electrochemistry, for example in batteries, fuel cells, photovoltaic devices and electrodeposition processes, for example in baths for the electroplating of metals.
Ionic liquids have very low vapour pressure and thus, in contrast to many conventional solvents, are very advantageous in that they produce virtually no hazardous vapours. They are therefore advantageous from a health, safety and environmental point of view.
One such system which has been known for many years is that formed from 1-ethyl-3-methylimidazolium chlorides-aluminum chloride (EMICxe2x80x94AlCl3). This system is a thermally stable liquid between xe2x88x92100xc2x0 C. to ca. 200xc2x0 C., defendant on the molar ratio of EMIC to AlCl3 utilised.
Such EMICxe2x80x94AlCl3 systems have been used extensively as solvents for various ionic reactions and as electrolytes, as described, for example in U.S. Pat. No. 5,525,567, FR-A-261170, FR-A-2626572, WO95/21872, and EP-A-838447 There are a number of difficulties in utilising such compounds. These arise principally from their cost, and from their water sensitivity.
In recent years, other ionic compounds have been made which are liquid at relatively low temperatures. For example, U.S. Pat. No. 4,764,440 discloses low temperature molten compositions, formed by reacting, for example, trimethylphenylammonium chloride with aluminum trichloride. The resulting ionic compound has a low freezing point (around xe2x88x9275xc2x0 C.), but suffers from the same water sensitivity as EMICxe2x80x94AlCl3, because of the presence of aluminium trichloride.
Proposals have been made to utilise other metal halides, in place of aluminum trichloride. For example, U.S. Pat. No. 5,731,101 discloses the use of iron and zinc halides as the anion portion of an ionic liquid composition. The cation portion is formed by an amine hydrohalide salt, of the formula R5 N.H.X. This reference indicates however that the aluminum compounds are preferred, and indeed contains comparative examples which indicate that it is not possible to substitute SnCl4 for aluminium trichloride. Furthermore, it does not suggest the use of quaternary ammonium compounds as cations.
FR-A-2757850 (equivalent to U.S. Pat. No. 5,892,124) discloses liquid salts of the general formula Q+Axe2x88x92, wherein Q+ represents quaternary ammonium or phosdhonium, and Axe2x88x92 represents a various anions including tetrachloro-aluminate, and trichlorozincate. It is suggested that such compounds are useful as vehicles for carrying out Diels-Alder reactions.
We have now found that by forming the anion of an ionic compound from a halide of zinc, tin, or iron, and the carson from certain specific quaternary ammonium compounds, it is possible to produce compounds which are liquid at relatively low temperatures (i.e. below 60xc2x0 C.), relatively inexpensive, and relatively water insensitive.
Accordingly, in a first aspect of the invention, there is provided an ionic compound having a melting point of no more than 60xc2x0 C., formed by the reaction of a quaternary ammonium compound of the formula
R1R2R3R4Nxe2x88x92Xxe2x80x83xe2x80x83(I)
or a mixture of two or more thereof;
with a halide of zinc, tin or iron, or a mixture of two or more thereof;
wherein R1, R2, and R3, are each independently a C1 to C5 alkyl or a C6 to C10 cycloalkyl group, or wherein R2 and R3 taken together represent a C4 to C10 alkylene group, thereby forming with the N atom of formula I a 5 to 11-membered heterocyclic ring,
and wherein R4, is a C6 to C12 alkyl or a cycloalkyl group, or a C1 to C12 alkyl or a cycloalkyl group substituted with at least one group selected from OH, Cl, Br, F, I, NH3, CN, NO2, OR5, COOR5, CHO, and COR5 wherein R5 is a C1 to C10 alkyl or cycloalkyl group, and
X as a suitable counter-ion.
The halide of zinc, tin or iron is preferably SnCl2, ZnCl2 or FeCl3, and it is generally found that the most favourable freezing point Is obtained when the molar ratio of the quaternary ammonium compound to the zinc, tin, or iron halide is from 1:1.5 to 1:2.2, preferably about 1:2.
A particularly surprising finding is that very low melting points can be obtained by employing as the anionic component a mixture of halides selected from zinc, tin, and iron, for example a mixture of ZnCl2 with SnCl2.
The quaternary ammonium compounds (I) used in the preparation of the ionic compounds according to the invention are asymmetric, in that they have at least one substituent group (R4) which is different from the remaining groups (R1, R2 and R3). R4 is preferably a C1 to C10 alkyl or a cycloalkyl group, substituted with at least one group selected from OH, Cl, Br, F, I, NH3, CN, NO2, OR5, COOR5, CHO, and COR5. It is particularly preferred that R4 is an ethyl group, substituted with one or more of hydroxyl, chlorine, or an ester (i.e. that the substituent R4 is derived from choline, chlorocholine, or a chlorocholine ester). Specific examples of R4 groups which have been found to be suitable are 2-hydroxyethyl, 2-bromoethyl, 2-chloroethyl, 2-acetoethyl, N-decyl, cyclohexyl, 2-hydroxy 3-chloropropyl, and 3-bromopropyl.
The counter-ion Xxe2x88x92 of compound (I) is preferably a halide, for example bromide or chloride, and will generally be the same halide as employed in the zinc, tin, or iron halide.
The ionic compounds according to the invention may be prepared simply by mixing together the quaternary ammonium compound (I), and the zinc, tin, or iron halide. The reaction is generally endothermic, and is usually carried out by heating, for example to a temperature of 100xc2x0 C. or more. No additional solvent is generally employed, although it may be advantageous in some circumstances to carry out the reaction in a solvent which is an ionic liquid, in particular, an ionic liquid in accordance with the invention.
The ionic compounds according to the invention may be utilised for a wide range of purposes, for example as electrolytes in electrochemical devices such as batteries or fuel cells, in photovoltaic or electrochromic devices, and in electrochemical deposition or electro-refining. The compounds find particular application for carrying out applications where a polar but non-aqueous solvent is required. They may also be employed as inert media, for dissolving ionic species such as transition metal complexes, and, either alone, or after complexing with other metal ions, as catalysts, or as chemical reagents.